RFR: 8270434: JDI+UT: Unexpected event in JDI tests

[Alex Menkov]

Updated the tests to skip events from unexpected threads.

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Commit messages:
 - Fixed test to handle events from system threads

Changes: https://git.openjdk.java.net/jdk/pull/5538/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5538&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8270434
  Stats: 28 lines in 3 files changed: 23 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5538.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5538/head:pull/5538

PR: https://git.openjdk.java.net/jdk/pull/5538